248 



K. MITSÜKURI. 



exteii.sion of the epiblastic area composed of columnar cells. In later 

 stages, (Figs. 6, 7, 8), the mass of the peristomal mesoblast no doubt 

 helps in causing opacity in the posterior region. From tlie stage of Fig. 

 6 on, the embryonic shield, which has hitherto passed gradually into the 

 surrounding jjarts, becomes sharply marked off from the circumjacent 

 transparent area, in which it is eccentrically placed, and becomes ap- 

 parently diminished in size. 



The -second point on which I wish to touch is as to the dorsal 

 opening of the invagination cavity, in the earliest stage 1 ])ossess 

 (referred to on p. 236-7 ), it is a squarish pit rather elongated in the 

 antero-posterior diameter. In Fig. 1, it is a wide open cavity 

 elongated transversely. In Fig. 2, it is a crescent-shaped transverse 

 slit, no longer gaping, and with its concavity turned forwards. The 

 same can be said of Figs. 3 and 4. In Fig. 5, the l)]astopore is 

 nearly straight across. It has a slight notch in the median line open 

 backwards. In Fig. 6, the ends of the slit-like opening have turned 

 backwards so that now the concavity faces backwards. In further 

 growth, the backward curvature becomes greater and greiiter, until it 

 becomes a h«3rse-shoe shaped slit, as can be seen in figures contained in 

 the former Contributions. Of the significance of this change of shape I 

 shall speak later on.* The yolk-plug, which can be traced more or 

 less clearly from the first, becomes very distinct as the backward 

 curvature becomes greater, and sticks out between the two limlis of 

 the horse-shoe. 



T'he yolk-[)lug in Fig. S is very peculiar in that it has a groove 

 in the median line. The cross sections of this reofion also show it to be 



* In his latest paper. Will (No. 21) seems to consider the enclosure of the primitive knol) 

 within the em1>ryonic shield as intimately connected with the change of the shape of the blasto- 

 pore-opeuing. Both are, according to him, due to the forward growth of the yolk-plug and the 

 consequent shoving forwards of the Wastopore-opening (see p. 127 et seq.). I find myself 

 imable to accept his views. 



